[MERGE] get_record_stream().get_bytes_as('chunked')

[Martin Pool]

We did have a case previously where a chunked-type interface made things
very slow, because a Python string can act as a list of chunks, causing
it to be split into characters and then reassembled.  The general style
a good one, I don't see a bug of that here, and I don't see a very good
place to eg assert something isn't a string.  Maybe we should add it to
the review guidelines.

-- 
Martin